One aspect of the present invention relates to a ribonucleoside substituted with a phosphonamidite group at the 3′-position. In certain embodiments, the phosphonamidite is an alkyl phosphonamidite. Another aspect of the present invention relates to a double-stranded oligonucleotide comprising at least one non-phosphate linkage. Representative non-phosphate linkages include phosphonate, hydroxylamine, hydroxylhydrazinyl, amide, and carbamate linkages. In certain embodiments, the non-phosphate linkage is a phosphonate linkage. In certain embodiments, a non-phosphate linkage occurs in only one strand. In certain embodiments, a non-phosphate linkage occurs in both strands. In certain embodiments, a ligand is bound to one of the oligonucleotide strands comprising the double-stranded oligonucleotide. In certain embodiments, a ligand is bound to both of the oligonucleotide strands comprising the double-stranded oligonucleotide. In certain embodiments, the oligonucleotide strands comprise at least one modified sugar moiety. Another aspect of the present invention relates to a single-stranded oligonucleotide comprising at least one non-phosphate linkage. Representative non-phosphate linkages include phosphonate, hydroxylamine, hydroxylhydrazinyl, amide, and carbamate linkages. In certain embodiments, the non-phosphate linkage is a phosphonate linkage. In certain embodiments, a ligand is bound to the oligonucleotide strand. In certain embodiments, the oligonucleotide comprises at least one modified sugar moiety.